Know your system
How you investigate depends on how your home heats water. A combi boiler heats water on demand, with no hot water cylinder. A system or regular boiler heats a cylinder of stored water, often in an airing cupboard, and many cylinders also have an electric immersion heater as a backup. Some homes rely on an immersion heater or electric water heater alone.
Combi boiler checks
These checks don't involve opening the boiler, which must only be done by a Gas Safe registered engineer.
- Is the boiler powered on and showing a fault code? Note the code for the engineer
- Check the pressure gauge – many systems run at around 1–1.5 bar when cold; the manual will say
- In freezing weather, check whether the condensate pipe outside has frozen
- Make sure the gas supply is on – are other gas appliances working?
- Try a reset, following the manufacturer's instructions, once
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Cylinder system checks
With a cylinder, check that the programmer is set to provide hot water and that the time and day are correct. Make sure the cylinder thermostat (often a dial strapped to the side of the cylinder) hasn't been turned down. If the heating works but the hot water doesn't, the motorised valve that sends hot water to the cylinder may have failed.
If you rely on an immersion heater, check its switch and the fuse or circuit breaker. A failed immersion element is a common fault, especially in hard water areas where scale builds up on it.
Who to call
For faults inside a gas boiler, or anything involving gas, call a Gas Safe registered engineer – it's a legal requirement. For an immersion heater element or wiring, use a plumber or electrician who is qualified to work on it. A plumber can deal with valves, cylinders, pumps, frozen pipes and leaks. Unvented cylinders must be worked on by someone qualified for unvented systems.
